Starch dispersions are used, among other things, as a binder in adhesive compositions and coating slips or sizes for paper. After evaporation of the major part of the water a dried starch layer is obtained which interconnects the constituents of the final product. In a number of cases the use of starch products in the above applications involves the drawback that the starch layer applied and dried is not water-resistant (waterproof). Under the influence of moisture and water the resulting bond will disintegrate after a short time.
A plurality of methods are known for preparing starch binders which, after having been applied to the carrier material and dried, are resistant to the action of water. The most important method of waterproofing starch binders is the addition of a reactive component (insolubilizing agent, waterproofing agent, cross-linking agent) to the starch dispersion. After the starch dispersion has been applied to the substrate, the desired waterproofness can be obtained by reaction of the insolubilizing agent with the starch molecules. The production of waterproof starch layers generally comprises the following three phases:
(a) Preparation of the adhesive composition or coating slip (hereinafter also called compositions) consisting of starch, water, waterproofing agent, and, if required, other components (catalyst, pigment, dispersing agent).
(b) Application of a layer of the adhesive composition between two substrates or of the coating slip to a substrate (in general paper and/or paperboard).
(c) Drying of the applied layer whereby a water-proof starch layer is obtained by reaction of the insolubilizing agent with the starch molecules.
Insolubilizing agents applicable in this respect are glyoxal, urea-formaldehyde resins, melamine-formaldehyde resins, ketone-formaldehyde resins and phenol-formaldehyde resins. These waterproofing agents each have their specific drawbacks with respect to odour, toxicology, pH conditions (required for obtaining sufficient waterproofness), degree of waterproofness and discolouration of the carrier material.
A general drawback of the above-described methods is the problem of durability (pot-life) of the adhesive compositions and coating slips or sizes before their application to the substrate. Prior to applying those compositions to the carrier material they have to be useful for a specific period of time. The fact is that there is a risk that, owing to premature reactions between the insolubilizing agent and the starch molecules, the viscosity of the compositions will undesirably increase. This viscosity may become so high that the compositions can no longer be processed by the machines used for applying these products to the substrate. The durability or pot life is affected in particular by the pH, the temperature, the nature and concentration of the insolubilizing agent and by the nature and concentration of the starch product. It is sometimes necessary to use starch products having very low viscosity and/or low starch concentrations. It may also be necessary to adjust the pH value of the binder in such a manner that no optimum waterproofing of the applied and dried starch layer is obtained. Furthermore, often suitable combinations of specific starch derivatives with specific waterproofing agents cannot be used, because their simultaneous presence too strongly limits the durability (pot-life) of the compositions before they can be applied to a substrate.